The present invention relates generally to an apparatus and method for remotely activating blasting devices. Such an apparatus and method may be used, for example, in oil and gas well production in other industries in which remote initiation of explosive devices occurs.
In the production of oil and gas from underground wells, it is known to convey a perforating gun on a wireline down a bore hole of a well to a position where an oil or gas bearing stratum is located, and then to detonate shaped charges in the perforating gun. The shaped charges penetrate the formation, facilitating the entry of oil or gas into the well.
Safe and reliable initiation of perforating guns or other firing devices in the well-bore, far removed from the surface, has been a continuing source of design challenges. The explosive train in the perforating gun normally comprises a detonator for setting off a detonating cord. The cord in turn detonates a series of connected shaped charges. The detonator is the first element in the explosive train and is normally the most sensitive to external stimulation. Generally speaking, the safety level of the perforating gun is primarily determined by the safety level of the detonator used. Bridge wire electric detonators have been, and are widely used. When an electric current of sufficient strength is applied to its lead wires the bridge wire is heated and ignites the pyrotechnic material surrounding it. This in turns sets off the primary and secondary explosive charges in the detonator.
An inherent problem with bridge wire detonators is the risk of unintentional detonation which may arise from stray currents. A bridge wire detonator does not possess the ability to distinguish between firing current and hazardous electric energy that reaches its lead wires. Typical sources of electrical interference which may cause unintentional detonation are communications equipment, whether cellular telephones or radio, standard 220V, 50 Hz or 110V, 60 Hz line current, electrostatic discharges and lightning. At present when bridge wire detonators are used for perforating jobs, typical safety measures include shutting down electric sources in the well rig environment and turning off communication facilities. It would be advantageous to provide the oil industry a method of initiating perforating guns and a detonator which reduces or eliminates the need to suspend the use of without suspending the electric devices and communication radio in the well rig environment. An additional problem concerns unauthorized use of the detonators. Lost, stolen or mishandled detonators that can be set off by commonly available power sources, whether deliberately or accidentally used, may pose a significant danger. It would be advantageous to have a detonator which will resist detonation except when initiated by an authorized person using a specially designed blasting machine.
A known approach to the problem of unintentional detonation is to add extra resistance in series with the bridge wire, making a xe2x80x9cresistorized detonatorxe2x80x9d. A higher voltage than would otherwise be required is used to fire a resistorized detonator, making it more difficult to set off. However, the magnitude of the electric current needed to initiate the detonator remains the same as non-resistorized detonators.
Another approach is to increase both the voltage and electric current needed to fire the detonator, so that they substantially exceed the upper limit of routine well rig electrical signals like the exploding bridge wire detonator or exploding foil detonator. This kind of exploding bridge wire or exploding foil detonator is disclosed in U.S. Pat. No. 4,777,878 of Johnson et al. issued Oct. 18, 1988 and U.S. Pat. No. 5,505,134 of Brooks et al., issued Apr. 9, 1996. Another approach, as shown in U.S. Pat. No. 4,708,060 of to Bickes et al., issued Nov. 24, 1987 and U.S. Pat. No. 5,503,077 of Motley issued Apr. 2, 1996, employs a semi-conductor bridge wire to achieve improved safety.
Still another method is to isolate the bridge wire, by employing a small transformer in the detonator. The load, generally the bridge wire of the detonator, is connected to the secondary winding of the transformer to form a loop and is electrically isolated from the primary winding of the transformer. The core material of the transformer is chosen to attenuate, or eliminate, spurious electrical power and radiofrequency signals and to respond to firing currents falling within a predetermined range of magnitude and frequency. A blasting machine provides electric current in the predetermined range needed to fire these inductive detonators.
A number of embodiments of transformer based detonators are shown in U.S. Pat. No. 4,273,051 of Stratton, issued Jun. 16, 1981. All of those embodiments employ some form of auxiliary energy dissipation means, whether a series or other leakage inductance, a fusible link, or a resistor in parallel with the primary winding.
Another example of a ferrite core, broad band attenuator is shown in U.S. Pat. No. 4,378,738 of Proctor et al., issued Apr. 5, 1983. U.S. Pat. No. 4,441,427 of Barrett, issued Apr. 10, 1984 discloses an oil well detonator assembly that uses ferrite materials to protect against radio frequency energy.
U.S. Pat. No. 4,544,035 of Voss, issued Oct. 1, 1985 discloses the use of two coils to initiate a detonator in a perforating gun without the coupling of magnetic materials. U.S. Pat. No. 4,806,928 of Veneruso, issued Feb. 21, 1989 discloses the use of coil assemblies arranged on ferrite cores for data transmission between well bore apparatus and the surface and which may also be used to fire perforating guns.
U.S. Pat. No. 3,762,331 to Vlahos, issued Oct. 2, 1973 discloses a firing circuit for detonators that uses a step down transformer having a voltage reduction of roughly 100:1 and a secondary coil having only 1 or 2 turns. It operates at a voltage between 60V and 240V and at a signal frequency of the order of 10 KHz. It is powered by a battery in parallel with a storage capacitor, which discharge through an inverter circuit which includes a solid state oscillator and a transformer for stepping up the resulting a.c. voltage to the desired level. This patent also discloses the use of shunt and series capacitance connected to the primary winding of the detonator, and a large step down at the detonator transformer. U.S. Pat. No. 4,145,968 to Klein, issued Mar. 27, 1979 describes primary and secondary windings and a fixed magnetic screen designed to be saturated in the presence of the magnetic flux generated by the primary winding. U.S. Pat. No. 4,297,947 to Jones et al., issued Nov. 3, 1981 discloses the use of a toroid or a magnetic core with removable parts as transformer cores to couple a relatively short (100 m) firing cable and a number of detonators.
U.S. Pat. No. 4,304,184 to Jones issued Dec. 8, 1981 discloses a transformer circuit whose primary and secondary windings are not completely isolated. Instead, they are coupled not only magnetically but also electrically. While this configuration may provide protection against hazardous electrical currents at low values and low frequencies, the safety features would be more satisfactory if the two windings were completely isolated electrically. None of the transformer-based detonators noted above appear to be suitable for oil well use.
A detonator that can be used in the oil industry at great depth poses special requirements for the coupling transformer. The electric energy supplied from the surface is transmitted along the wireline cable down oil wells as deep as 7,500 m. The cable used for well logging and casing perforation may not be designed for high frequency transmission. The distributed shunt capacitance along the cable is in the order of 0.15 uF/Km. The attenuation for high frequency electrical energy is as high as 3 db/Km (at 20 KHz). Consequently, for effective power transmission along the wireline, a relatively low frequency is preferred. However, electric currents having a frequency lower than 1 KHz will be attenuated by the ferrite core transformer and may not yield a suitable output for energizing the bridge wire in the secondary winding. Therefore, frequency significantly higher than 1 KHz is preferable and the blasting machine must be powerful enough to allow energy dissipation along the wire-line and still secure reliable initiation of the detonator. For optimum power transmission, the inductance of the transformer used in the detonator must be in a certain range at a certain firing current frequency. The inductance of a transformer of some typical known designs may fall in the range of 1-50 xcexcH. Inasmuch as the characteristic impedance of a typical monocable used in well logging is about 30-50xcexa9, usable for oil well wirelines.
By contrast, a transformer having a relatively high primary inductance in the order of 40 mH, would be unsuitable even at the lowest usable frequencies. Also, where the step down is too large, the relatively high voltage needed to fire the detonator makes it impractical for oil well use because of the rapid attenuation of the high frequency voltage signal along the cable. In the view of the inventors of the present invention, the preferred frequency range for effective power transmission is between 3 and 20 KHz, and the primary inductance of the transformer should be in the range of 200 xcexcH and 3 mH.
A number of the transformers noted above use magnetic cores which provide a closed magnetic circuit. Some of them may have removable parts to accommodate the firing cable and detonator legwires, as disclosed by U.S. Pat. Nos. 4,297,947 or 4,601,243. When the primary inductance needed is small and a relatively big transformer core (for example, a toroid having outer diameter of 20 mm, placed outside the detonator body) is used, a few turns of winding may be sufficient. However, for a higher impedance the number of winding turns is relatively large, normally in the range 15-80 for the primary winding, depending on the actual size and material properties of the transformer core. Generally the core size of the transformer should be comparable to that of the outside diameter of the detonator. For an oil well detonator this dimension is commonly about 6-7 mm. In the view of the present inventors, as a practical matter, it is difficult efficiently to wind such a large number of turns on a small transformer core, such as a toroid.
In the view of the inventors, some of these difficulties may be addressed by using a transformer constructed with a simple core in the form of a column having the desired number of primary and secondary windings on it. A column represents an open magnetic circuit. To achieve efficiency in manufacturing, especially in mass production, it would be advantageous to form the primary and secondary windings by winding separate coils, and then be assembling those coils onto the column shaped core. Alternatively, the primary and secondary windings could be wound on a simple machine sequentially, with the primary winding be embedded, or nested, within the secondary winding, or vice versa. Different shapes of the column can be used, such as a square column, a plate, a tube, a U-shaped core, or other suitable form.
In an open magnetic circuit, there is energy loss associated with the high magnetic resistance. It would be advantageous to reduce this loss by using another piece of magnetically permeable material to form a closed magnetic circuit transformer core. Examples of such materials are nickel-iron alloys or permalloys and silicon steel, which have a high magnetic permeability, high curie temperature and are small in volume, low in cost and flexible to form different shapes as required.
The oil well use of a transformer-based detonator presents technical challenges. In addition to the extremely long transmission distance (up to 7,500 m long) discussed previously, the high temperature environment also tends to present design challenges. Firstly, magnetic permeability of the core changes with increases in temperature, and drops to near zero above the Curie temperature. Magnetic materials lose their magnetism and the ability to transmit signals beyond the Curie temperature. Advantageously, magnetic materials chosen for transformer cores should have a Curie temperature higher than the highest anticipated temperature in the well, typically 180xc2x0 C. or higher. Secondly, the ability of most magnetic materials to transmit energy decreases substantially with the increase in temperature due to the decrease in saturation flux density. For example, for a typical maganese-zinc ferrite material, the saturation flux density at room temperature is 4500 Gauss. This decreases to 1750 Gauss when the ambient temperature is 200xc2x0 C. It is advantageous for the transformer detonator to be able to transmit the required amount of initiation energy at reduced saturation flux density. Thirdly, for ferrite materials there is generally an optimum temperature point at which the core loss at a minimum. Deviation in temperature from that point would result in increased core loss. Even though the detonation location well temperature may vary, it is advantageous to choose a ferrite material which has an optimum core loss temperature close to the expected well temperature.
A blasting machine is an electronic device which sends a high frequency electric signal through the wireline to fire the detonator. It is advantageous to provide a blasting machine whose output characteristics match the preferred frequency range of the detonator.
U.S. Pat. No. 4,422,378 discloses an ignition circuit for firing detonators having a toroid transformer. It uses a power oscillator having a transistor to provide a firing signal at the resonant frequency of a network of detonators, the transistor being controlled by a current feedback signal. This self-adjusting resonance matching is possible when the inductance and capacitance of the detonators connected in a net are detectable. In some applications, such as those in which diodes are placed in series with the wireline, the inductance of the line can not be obtained and it is difficult automatically to generate the resonant frequency.
U.S. Pat. No. 4,422,379 discloses another ignition circuit for firing detonators with a toroid transformer. The oscillator of the circuit is a typical push-pull power amplifier with the use of an output transformer. U.S. Pat. No. 4,848,232 also uses a firing circuit in the form of a push-pull power amplifier with an output transformer.
In U.S. Pat. No. 4,601,243, the electrical charge stored by a capacitor is discharged to detonators through a high frequency converting unit which oscillates at a frequency between 50 KHz and 1 MHz.
The above referenced U.S. patents commonly have an output transformer. It would be advantageous to eliminate the use of such an output transformer in the blasting machine. First, power output tends to be limited by the size of the transformer. Long transmission distances or initiation of many detonators in one round tends to require a relatively big transformer. This weight and size disadvantage tends to be more pronounced at relatively lower frequencies such as the 3-20 KHz range noted above. When a large, heavy transformer is used the manufacturing cost also tends to increase.
It would be advantageous to have an electrically activated detonator operable at great distances, from an electrical signal source, such as may be desired for perforation of an oil well thousands of meters from the surface.
It would be advantageous to have a simplified, electrically activated detonator that is relatively insensitive to signals from common electrical sources such as radios, telephones, 50 and 60 Hz supply signals, and other stray or static signals.
It would be advantageous to have a blasting machine for activating remote detonators that does not require the use of a large, heavy, and expensive iron core output transformer.
The present invention provides, in a first aspect, a detonator for igniting explosive material comprising a multi-turn primary coil for connection to a detonation signal source; a multi-turn secondary coil connected to an explosive igniting element; and a core magnetically linking the coils. The core has a mandrel upon which at least one of the coils is mounted.
In a second aspect of the invention there is a detonator for use in a well perforating gun comprising a transformer having a pair of multi-turn coils linked by a magnetically permeable core. The core has a mandrel. One of the coils is a pre-formed coil mounted upon the mandrel. One of the coils is connectible to a detonation signal source and the other coil is connected to an explosive igniting element with which it forms a closed circuit. Explosive material is in contact with the explosive igniting element.
The invention may also have a magnetically permeable closure member fit to the mandrel to form a closed loop magnetic circuit. Each of the coils may be a pre-formed coil. Each of the coils may be mounted on a mandrel of the core. The detonator may have closure member fit to each mandrel to form a closed loop magnetic path.
In a still further aspect of the invention there is an assembly for causing an explosive charge to explode comprising a blasting machine for generating a detonation signal; a detonator for receiving a detonation signal; and a carrier for carrying a detonation signal from the blasting machine to the detonator; the detonator having a transformer having a pair of multi-turn coils linked by a magnetically permeable core, one of the coils being connectible to the signal carrier; an explosive igniting element connected to the other coil to form a closed circuit; explosive material in contact with said explosive igniting element; and the core having at least one mandrel, and at least one of the coils being a pre-formed coil mounted on the mandrel.
In a further aspect of that invention, the blasting machine of the explosive assembly further comprises an energy storage system; a discharge system for releasing energy from the storage system; a switching system operable to control the discharge system to release the detonation signal from the energy storage system for communication of the signal to the detonator along the carrier.
In an even further aspect of the invention there is a blasting machine for producing a specific signal for setting off a signal selective detonator, comprising a charge storage system; an output port for connection to the signal selective detonator; a switching system connected between the charge storage system and the output port; a pre-set discharge control system operable to vary flow of charge through the switching system to produce the specific signal.
In further aspect of that even further aspect of the invention, the blasting machine further comprising a charging system selectively connectible to the charge storage system when the discharge control system is inoperative.
In another further aspect of that even further aspect of the invention the charging system includes a transformer connectible to draw power from a standard line source, and a rectifier connected to the transformer for converting the power to a form storable in the charge storage system.
In yet another aspect of the invention there is a detonator for igniting explosive material comprising a primary winding for connection to a detonation signal source; a secondary winding and an explosive igniting element connected thereto; and a core magnetically linking the primary and secondary windings. The core has a first portion made from a first magnetically permeable material for attenuating signals in a first frequency range, and a second portion made from a second magnetically permeable material for attenuating signals in a second frequency range.
In a still further aspect of the invention a detonator for igniting explosive material comprises a multi-turn primary coil for connection to a detonation signal source and a multi-turn secondary coil and an explosive igniting element connected thereto. The coils are co-axially mounted and magnetically coupled by a core of low magnetic permeability.